1. Field of the Invention
The present invention relates to an image capturing device that is provided with a light source and a camera and in which a measurement subject is placed in an optical path that lies therebetween, in particular, to an image capturing device that captures an image of a measurement subject in synchronization with ON/OFF states of the light source.
2. Description of the Related Art
Image capturing devices that are provided with a light source and a camera and in which a measurement subject is placed in an optical path that lies therebetween are known. In these image capturing devices, when an image of a measurement subject is captured in synchronization with the ON/OFF states of the light source (so-called lock-in image capturing), not only images caused by radiations other than the light source can be eliminated, but also low-frequency noises such as 1/f noises can be set to the off setting.
For example, as presented in Non-patent Literature 1 (A. W. M. Lee et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL 18, NO. 13, Jul. 1, 2006, p. 1415-1417), when a THz wave emitted by a THz light source that is periodically turned ON and OFF is transmitted to a camera whose sensitivity is within a range from the infrared region to the THz (terahertz) region, the camera detects an electromagnetic wave in which infrared and a periodic THz wave have been mixed and captures an image based on the detected electromagnetic wave. By calculating the difference between image data captured by the camera during the ON period of the THz light source and image data captured by the camera during the OFF period thereof, infrared images can be set off and thereby only THz images can be obtained. As a result, a filter that suppresses infrared waves and passes through only THz waves can be omitted.
A specific arrangement of the foregoing image capturing device is shown in FIG. 1.
As shown in FIG. 1, light of a THz wave at a frequency of 4.3 THz (with a wavelength of 70 μm) is emitted by QCL (Quantum Cascade Laser) 102 that is mounted on cooler 101. The light travels to off-axis parabolic mirror 103. Off-axis parabolic minor 103 collimates the light and reflects the collimated light to off-axis parabolic mirror 104. Off-axis parabolic mirror 104 radiates the collimated light to envelop (measurement subject) 105. The light that has passed through envelop 105 is collected by Si lens 106 to microbolometer array sensor 108 that has 320 ×240 pixels and that is mounted on microbolometer camera 107 (at a frame rate of 60 Hz). Microbolometer array sensor 108 captures an image based on the collected light of the THz wave.
FIG. 2 shows a lock-in image capturing method for the image capturing device shown in FIG. 1.
As shown in FIG. 2, QCL 102 is turned ON at frame 1 and QCL 102 is turned OFF at frame 2 and frame 3.
At frame 1, an infrared wave and THz wave emitted by QCL 102 are collected to microbolometer array sensor 108 and thereby an image is captured based on both the infrared and THz wave, whereas at frame 2 and frame 3, only infrared waves are collected to microbolometer array sensor 108 and thereby an image is captured based on only the infrared.
Thus, by calculating the difference between the image data of frame 1 and the image data of frame 3, a THz image can be obtained as an image captured based on only the THz wave emitted by QCL 102.
Frame 2 is not used in order to increase the difference between image data of frame 1 and image data of frame 3 since a sufficiently long time elapses after the image data of frame 1 has been captured compared to a thermal time constant of around 13 msec of microbolometer array sensor 108.
FIG. 3 shows THz image 110 that is captured from envelop 105 that contains paper 109 with letters “MIT” written with a pencil at a frame rate of 20 Hz based on the method shown in FIG. 2. Thus, letters “MIT” can be checked out without necessity of opening envelop 105.
However, in the foregoing image capturing device, since only the difference between one piece of image data captured during the ON period of the light source and one piece of image data captured during the OFF period thereof, as a problem that would arise, the signal-to-noise ratio could not be improved as expected.